There have been various approaches on how to perform WiMax network entry, a brief overview of which is presented hereafter.
In WiMax TDD (“Time Division Duplex”), the frame structure comprises a preamble, followed by a downlink (DL) transmission period and an uplink (UL) transmission period with respective intervals of time inserted in between, to allow the mobile station to switch from receiver mode to transmitter mode and back. The frame duration in mobile WiMax of 10 MHz bandwidth is 5 ms (fixed). During this time interval, 48 OFDMA symbols are transmitted in the frame. The structure of the frame is presented in relation with FIG. 1.
The preamble of the frame comprises an OFDMA symbol which repeats itself every frame length (5 ms) and it serves as a means of synchronization of a mobile station (MS) to a base station (BS). The useful subcarriers of the OFDMA symbol which make up the preamble (ex: 852 useful subcarriers for an FFT size of N=1024 and spaced at a fixed distance of 10 kHz, no matter the value of N) are modulated one over three using a boosted BPSK modulation with a specific pseudo-noise (PN) code. Depending on the starting offset used to modulate the subcarriers one over three (0, 1 or 2 subcarriers), there are three defined segments. The standard states that there are 114 possible PN patterns for a WiMax preamble, i.e 38 per segment.
In order to enter the network, the mobile station must first scan among all the couples (central frequency/bandwidth) candidates, in search of one of the 114 preambles. In other words, in WiMax, as in other OFDM systems, the classical approach for mobile station (MS) synchronization comprises testing (scanning) all frequency/timing hypotheses over all couples of (central frequency/bandwidth) candidates, calculating a cross-correlation in the time domain for each such hypothesis, followed by peak(s) detection.
After a synchronization phase, the actual preamble detection is performed. The issue with these types of time domain algorithms is that although they offer very good accuracy, they are computationally expensive because of the exhaustive hypothesis testing.
A list of couple (central frequency/bandwidth) candidates can be, for example purpose: ((3.5 GHz, 10 MHz), (3.5 GHz, 5 MHz), (3.6 GHz, 10 MHz), (3.6 GHz, 5 MHz), . . . ).
The same approach of exhaustive scanning can be equally done in frequency domain.
An alternative to this technique would be to perform a blind autocorrelation in time domain (without any prior hypothesis testing). Because of the structure of the WIMAX preamble, which presents a factor three decimation in the frequency domain, a three peaks detection can be done in time domain which provides the synchronization information. This method works well when dealing with an AWGN channel, but dealing with a non line of sight channel would oblige long time averaging which involves a longer network entry phase.
Regarding the preamble detection, which is done post synchronization, a differential approach in frequency domain is also used to compensate the frequency selective fading, since the cell search is performed before channel estimation.
A conventional approach for accelerating network entry through PHY layer algorithms is to perform ordering of the candidates by means of power detection prior to synchronization. Although this filtering phase may be useful for reducing the list of possibilities, it does not guarantee that the detected power corresponds to a WIMAX signal (other communication modes may be in the frequency vicinity of a WiMAX signal). This can slow down network entry by attempting mobile station (MS) connections to false candidates (for example access points or base stations with other communication modes, such as a WiFi access point).
Acceleration of the network entry is however typically done in higher levels by using heuristics (for instance an ordering of candidates by most recently used, performed in the MAC level), but a procedure for doing this acceleration in the PHY layer does not exist.